Chapter 7 Electricity

Electric Charge There are 2 types of electric charges. Protons have a positive charge Electrons have a negative charge The amount of electric charge in a proton and an electron is the same! (write this!) In most objects, the positive and negative electric charges balance each other out. They are called neutral. Static Electricity—the accumulation of excess electric charges on an object.

Conservation of Charge Law of Conservation of Charge—charge can be transferred from object to object, but it cannot be created or destroyed.

Conductors & Insulators Conductor—material through which an excess of electrons can move easily Metals are excellent conductors of electricity. The electrons in metals can move easily through the material. Electric wires are usually made of metals such as copper. Insulator—material that doesn’t allow electrons to move through it easily Plastics, wood, rubber, and glass are good insulators.

Transferring Electric Charge Objects can become charged in many ways. Charging by contact—the process of transferring charge by touching or rubbing. Think of static electricity Charging by induction—The rearrangement of electrons on a neutral object caused by a nearby charged object.

Lightning & Thunder Lightning is a large static discharge. A static discharge is a transfer of charge through the air between 2 objects because of a buildup of static electricity. Lightning also generates powerful sound waves. The energy in the lightning pulls electrons off atoms in the atmosphere producing heat. The heat warms the air; the air expands & produces sound waves (thunder)

Detecting Electric Charge Electric charge can be detected by an electroscope. An electroscope is made of 2 thin metal leaves attached to a metal rod with a knob at the top. The leaves hang straight down when not charged. When charged, the leaves will move apart.

Sec 2: Electric Current Charges can move or flow through certain materials. Charges flow from high-voltage areas to low-voltage areas. Voltage difference (V)—the push that causes charges to move. (measured in volts) For charges to flow, a wire must be connected in a closed loop, or circuit—a closed conducting path. Electric current ( I )—the flow of charges through a wire or any conductor. Measured in amperes (A)

Batteries In order to keep current flowing, there must be a source of voltage difference. A battery is a common source of voltage difference. Dry-Cell batteries—A zinc container surrounds a chemical paste with a solid carbon rod in the middle. Ex. AA, AAA, C, D Wet-Cell batteries—contain 2 metal plates in a conducting solution. Ex. Car Battery Voltage difference can also come from electrical outlets.

Resistance Electric current loses energy as it moves through a light bulb because the light resists the flow of electrons. Resistance—the tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light. Measured in ohms (Ω) Most materials have some electrical resistance. The size of a wire affects its resistance. Thinner wires and longer wires have more resistance.

Controlling the Flow of Electrons There are 2 ways to control the flow of electrons, or electric current. Adding voltage difference increases current Adding resistance decreases current.

Ohm’s Law Ohm’s Law—the current in a circuit equals the voltage difference divided by the resistance. Current = voltage difference resistance I = V/R or V = I R The unit for current (I) is the ampere (A) The unit for voltage difference (V) is volts (V) The unit for resistance (R) is the Ohm (Ω)

Electrical Power & Energy Electrical Power- the rate at which electrical energy in converted into another form. Electric Power (watts)= Current (amperes) Voltage Diff (Volts) Electrical Energy Equation: Electrical energy(kWh)= electric power(kW) x time (hrs) kWh= kilowatt hours kW=kilowatts

Sec. 3 Electrical Energy To use electrical energy, a complete circuit must be made. 2 Types of circuits are series & parallel Series circuit—the current has only one loop to flow through. Current is measured with an Ammeter, and must be connected in series with the thing you are measuring

Parallel Circuit Parallel circuits—contain 2 or more branches for current to move through. The current splits up to flow through the different branches. Voltage (or voltage difference, V) is measured with a voltmeter. A voltmeter must be in parallel with the thing you are measuring.

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